1. Field of the Invention
The present invention relates to an optical information recording medium for use in an information recording/reproducing apparatus making use of a laser beam, particularly to a rewritable optical disk.
2. Description of the Related Art
A phase-change type optical disk has been known having a recording film made of chalcogenide which enables recording, reproduction and erasing of information signals.
In ordinary erasable phase-change type optical disk apparatus, signals are recorded in a recording medium in an amorphous state, while a crystalline state of the medium is obtained after erasing of signals.
Materials containing Te or In as major components have been practically and broadly used as the material of recording films on recording mediums of the kind described. In particular, a thin film of a Ge-Sb-Te type ternary alloy can be crystalized in quite a short period of 100 nsec or shorter upon irradiation with light together with heat, provided that the composition is suitably selected This type of alloy is disclosed in Japanese Patent Unexamined Publication No. 62-209742. In general, a shorter crystallization time, i.e., a shorter erasing time, contributes to a higher signal transfer rate. Thus, theoretically, a very short crystallization time enables over-writing with a single beam, namely, simultaneous writing and erasing of signals by a single laser beam with a suitable intensity modulation. In fact, a phase-change type optical disk having a recording film of a Ge-Sb-Te type ternary alloy of a suitable composition enables over-writing with a single beam, by virtue of its very high crystallization speed, as discussed in Japanese Patent Unexamined Publication No. 62-209742.
The number of repetition of recording and erasing of information in a phase-change type optical disk can be increased by suitably selecting and optimizing conditions such as the material of the recording film construction of the disk, levels of the recording power and erasing power, and so forth. However, optical disks are not always used under optimum conditions. In particular, optical disks for personal use are used under various conditions. Thus, it is desirable that optical disks are used with high reproducibility of operation characteristics even when used under a varying condition such as offset of the power from an optimum level.
The present inventors have conducted experiments for the purpose of examining how the performance of optical disks having recording films mainly composed of Te or In are influenced by deviation of the recording and erasing power levels. As a result, the inventors have found that the deviation of the power levels from optimum levels tends to lead to destruction of the recording film and limitation in the number of cycles of repetition of recording and erasing.